In contemporary communications systems, it is known for payload data, e.g. relating to voice, video or multimedia communication, to be transmitted via packet-oriented networks. The use of packet-oriented data transmission methods frequently results in losses in the quality of the payload data being transmitted, such losses frequently being due to conversions and delays of payload-data packets in network nodes.
Based upon the frequently used Internet Protocol, also called IP for short, communication terminals which operate on the basis of a packet-oriented transmission technology will also be referred to below using the term “IP terminal”. In the packet-oriented transmission of payload data, the terms “Voice over IP”, VoIP for short, and “Multimedia over IP”, MoIP, are also commonplace.
In the packet-oriented network, a connection between IP terminals is normally routed via network elements such as routers, switches or gateways. A distinction is frequently made between signaling connections and payload-data connections. Here, routers normally switch data between packet-oriented networks, while gateways link different networks.
The network elements and IP terminals may comprise functions for the coding—also called encoding—and decoding of payload data by means of codecs (codec: coding and decoding). At the same time, a network element or an IP terminal each frequently comprise several codecs, in particular for different types of payload data, for different bandwidths and for different levels of payload-data quality after decoding.
Payload data, also called payload, includes e.g. voice/sound, video or multimedia data. Codecs frequently used for voice encoding include e.g. the non-compressing G.711 codec or the compressing G.729 codec of the ITU (International Telecommunications Union). Codecs for video encoding include e.g. the MPEG-1-Video codec or MPEG-2-Video codec of the MPEG committee (MPEG: Motion Picture Experts Group).
A communications system is frequently characterized by having a structure comprising individual subnetworks, the subnetworks frequently being linked to one another via gateways. Payload data and signaling data can be transmitted between an IP terminal and a gateway, as well as between two gateways respectively, it being possible for a specific transmission method to be selected for each of these sub-connections—e.g. a first sub-connection from a first IP terminal to a gateway and a second sub-connection from said gateway to a second IP terminal. This selection is based in particular on the codecs available in the respective IP terminals and gateways and on the transmission protocols provided.
In the case of a connection between two IP terminals, the payload data can be routed via several gateways, particularly where the communication connection extends over several subnetworks. Usually, incoming payload data is buffered in each gateway by means of a jitter buffer, decoded with a codec selected for an incoming sub-connection and, for forwarding of the payload data, encoded with a further codec selected for a further sub-connection. This can lead to a delay of the payload data, in particular as a result of the buffering. Furthermore, the encoding and decoding can lead to a deterioration in the quality of the payload data, particularly if compressing codecs are used, and consequently, during each conversion, i.e. encoding and decoding, payload-data information may be lost.
Furthermore, in many cases, multiple conversion into a transmission method or protocol supported by the respective subnetwork is required, particularly if the IP terminals and the gateways do not support the same encoding method. This may lead to a deterioration in the quality of a connection as a result of multiple decoding and encoding, data compression in the sub-connections and use of a codec not suited to the bandwidth available.
Normally, when setting up a connection between two IP terminals linked via a gateway, for sub-connections from the first IP terminal to the gateway and from the gateway to the second IP terminal, in particular an encoding method of the communications partners for the respective sub-connections is exchanged and negotiated by means of signaling and/or acknowledgment messages. Upon connection acceptance by the called second IP terminal, the payload data is transmitted via the first and second sub-connection between the respective communications partners of these sub-connections with the negotiated encoding method for the respective sub-connections, buffering and decoding and encoding of the payload data being carried out in the gateway with the respective encoding method for the sub-connections.
In known methods for the transmission of payload data, the IP terminals and/or the gateways, as communications partners of a sub-connection, have to select the same or compatible codecs for encoding and decoding, in order that the receiving communications partner can decode the received encoded data. Here, compatible codecs means that encoded payload data of the communications partner can be decoded—possibly with a loss of quality.
To rectify at least some of the disadvantages stated, IP terminals which support the ITU-T recommendation H.323 (ITU-T: International Telecommunication Union —Telecommunications Standardization Sector) and Direct Media Connection (DMC) can transmit communications payload data directly between said IP terminals, bypassing the gateways and thereby avoid additional conversions and delays. However, this is only possible if both IP terminals support the same encoding method.